NASA says 1,284 new planets found outside our solar system by Kepler telescope

NASA has announced the discovery of 1,284 new planets outside our solar system, more than doubling the number of known exoplanets found with the Kepler space telescope.

"This gives us hope that somewhere out there, around a star much like ours, we can eventually discover another Earth," said Ellen Stofan, chief scientist at NASA headquarters in Washington.

The unmanned Kepler space observatory, which launched in 2009, has been scanning 150,000 stars for signs of orbiting bodies, particularly those that might be able to support life.

It works by observing a dimming in the light of a star, known as a transit, each time an orbiting planet passes in front of it.

"Of the nearly 5,000 total planet candidates found to date, more than 3,200 now have been verified, and 2,325 of these were discovered by Kepler," NASA said in a statement.

Of the new trove, nearly 550 could be rocky planets like Earth, based on their size, the US space agency said.

"Nine of these orbit in their sun's habitable zone, which is the distance from a star where orbiting planets can have surface temperatures that allow liquid water to pool."

The addition of these nine means that 21 exoplanets are now known to be possibly circling their stars in the habitable zone, and may harbour life.

However, Kepler is a "statistical mission", NASA scientists said, and is not designed to probe further into the conditions of certain planets that exist in the so-called "Goldilocks zone" of their stars — neither too hot nor too cold to sustain life.